This invention relates to a coupling system or assembly for interconnecting a drive shaft, a driven shaft and a servo-motor which normally powers the driven shaft. This mechanism is particularly useful in an automotive vehicle power steering system.
In general, a power steering system for an automotive type vehicle includes a drive shaft or steering column which is controlled by the vehicle operator turning the steering wheel, and a power assist device for the driven steering system. Hydraulic devices are conventionally used to provide the power assist. In addition, it is known to use a servo-motor arrangement to provide the power assist for the drive. An example of the use of a servo-motor arrangement is disclosed in U.S. Pat. No. 3,893,534 issued July 8, 1975 to Steinmann.
In any power assist device used in a power steering system, there is a danger of malfunctioning or power failure. Although failures are rare, it is necessary to provide a direct drive alternative to the power assist drive during times of malfunctioning or failure. When the servo-motor resists or opposes the action of the driver, the servo-motor should be disengaged, whereas it should remain engaged if it provides some assistance even though it may be in a state of partial failure. Prior systems have lacked these qualities to some degree or have had other deficiencies which have kept them from achieving commercial success.
Thus, this invention describes a coupling system which provides a fail-safe connection between the servo-motor power source and the drive and driven shafts or members to allow a direct drive in the event of power failure. During such direct drive the power source is prevented from interfering with, but is allowed to assist the direct drive.